Here are my not-so-unbiased-opinions. If you are concerned about getting plenty of hot water at a reasonable price, don't walk away from tankless...RUN! Most likely you do not have a gas supply sufficient to operate it, so big $$$ to take care of that problem. In those cold New England winter months, you will not have hot water, only warm. Whoever told you power vents were unreliable needs a power vent inserted. They are more costly, but in my experience, they are quite reliable. I would suggest that you need a large tank. A 40 gallon tank is fine for a small family, but you are using a considerable amount. Of course an 80 gallon tank will cost more than a 40 gallon, but way less than a tankless. Turning the thermostat up does not provide more water in the tank, it just makes what you have hotter so when it is diluted with cold water you will realize a bit more very warm water, but that's not a good solution to you problem. You need more capacity. Others might be able to give you better advise about connecting another 40 gallon tank to the one you have, and that might be a better solution. I'm not really well enough informed about that to offer a strong opinion. It would give you more capacity, but beyond than I can't say.